A tolled Transmission Gully Motorway should be the only route available to trucks to relieve pressure on the coastal highway, urges a Porirua city councillor.

Ken Douglas, a former head of a truck drivers' union, said he supported a controversial proposal to toll the inland highway.

But, if transport companies do not force their drivers to use the Gully route, local authorities such as Porirua City Council will end up paying the price, he said at a committee meeting last week.

"We should be insisting that trucks use Transmission Gully, otherwise we are going to have to maintain the coastal road, which will be hit hard. I used to represent truckies but I don't think there should be a free decision on which road you can use."

Meanwhile, the Government's decision last week to finance the $1 billion project through a public- private partnership (PPP) has been greeted with praise and scorn across the political divide.

A PPP is a long-term contract, involving Government and private funding, that National says will provide certainty around financing, construction and operation of the project.

Mana MP Kris Faafoi said it would be more prudent to see the cost benefits of a PPP compared to a solely Government-funded proposal.

"We don't know what this [PPP] approach is based on, it throws up questions about the motivation of the Government doing something that will make private profit or really benefiting the public.

"Any business case has to be solid and you have to remember this is Wellington, not Brisbane or London, so how do you spread the risk? What we don't want is a failed venture and a white elephant on our hands."

Mr Faafoi said the level of tolling could again come down to the private interests in a PPP.

NZ Transport Agency chief executive Geoff Dangerfield said a PPP "opens the door for private sector innovations", while stressing full ownership of the highway would remain in public hands.

A short-list of companies would be made in mid-2013 and the contract awarded in 2014, with construction expected to take six years.

Mr Dangerfield said tolling, to be investigated next year, would offset annual costs and meant other projects could be progressed.

The Greens' transport spokeswoman Julie Anne Genter said a PPP was an expensive form of borrowing - with interest paid back over 25 years - and an "irresponsible use of taxpayer money".